The inkjet recording method is a method of ejecting an ink droplet from a fine nozzle head and recording a letter or a figure on the surface of a recording medium such as paper. The inkjet recording method practically used includes a method of converting electrical signals into mechanical signals by using an electrostrictive element and intermittently ejecting an ink droplet stored in the nozzle head section, thereby recording a letter or a symbol on the recording medium surface, and a method of generating a bubble by abruptly heating a part of the ink solution in a partial portion close to the ejection portion of a nozzle head and intermittently ejecting an ink droplet as a result of volume expansion generated by the bubble, thereby recording a letter or a symbol on the recording medium surface.
As for the inkjet recording ink, an aqueous pigment ink obtained by dispersing a pigment in water has been recently provided, because an ink using a pigment can give a printed image excellent in water resistance or light fastness as compared with an ink using a water-soluble dye. In such an aqueous pigment ink, a pigment is generally dispersed in an aqueous dispersion medium by using a dispersant such as surfactant or polymer dispersant. However, the dispersant merely adsorbed to the pigment surface sometimes deteriorates the dispersed state by repeating desorption and adsorption.
For the purpose of enhancing the dispersibility of pigment contained in the inkjet recording ink, a technique of using a microencapsulated pigment obtained by covering a colorant particle with a polymer is known. For example, there are disclosed a microencapsulated colorant dispersion and an aqueous ink composition, comprising a micro-encapsulated colorant obtained by dispersing a water-insoluble colorant in the presence of a vinyl-based polymerizable dispersant and a polymer azo initiator containing a specific repeating unit under the conditions of not causing activation of the polymer azo initiator, and then activating the polymer azo initiator in the obtained dispersion to polymerize the vinyl-based polymerizable dispersant (see, for example, Patent Document 1).
In the inkjet recording ink, a metal powder having a metallic luster (hereinafter sometimes simply referred to as a “metal powder”), such as aluminum, and if desired, a pigment comprising a coloring material for imparting other arbitrary color tone (hereinafter sometimes-referred to as a “metallic pigment”) are sometimes used.
However, metal powder and the like easily available on the market are low in the luster due to surface non-uniformity originated in the production process or because of their high reactivity, restricted in view of use, for example, the metal powder and the like must be stored in an inert gas atmosphere away from contact with water or the like. Furthermore, the reactivity with moisture in air is also high and this gives rise to a problem in the storability, for example, the quality deteriorates during a long-term storage due to generation of hydrogen or gelling. In addition, the metal powder is not easily blended with a coloring material for imparting an arbitrary color tone, and desired coloration is difficult to attain.
The highly active metal powder such as aluminum readily reacts with water in an aqueous solvent and can be hardly applied to an ink using an aqueous solvent, because problems arise during storage, such as generation of hydrogen or gelling. The metal powder has been heretofore usually used for a non-aqueous solvent-type ink.
Furthermore, in the case of storing a conventional metallic pigment or an ink, coating material or the like containing the metallic pigment, these cannot be stored for a long time because of problems of reactivity with water and surface oxidation.
In addition, the metal powder for use in conventional metallic pigments usually has a particle size as large as 20 to 30 μm and cannot be used as it is for the inkjet ink.
Not only the particle size but also the specific gravity is large and therefore, the metal powder is bad in the dispersion stability and readily precipitates. The dispersion stability cannot be satisfactorily improved even by using the above-described microencapsulation technique.
On the other hand, in order to impart metallic luster/brightness decoration or the like to an image obtained by the inkjet ink, an inkjet ink containing flakes of a laminated body having a metallized layer and a resin layer is disclosed (see, for example, Patent Document 2). However, the technique of Patent Document 2 cannot overcome the problem regarding reactivity with water.
Patent Document 1: JP 2003-113327 A
Patent Document 2: JP 11-343436 A